Two questions are raised here: Why don’t electrons become other matter and why don’t they just decay into energy beams?
The first question is simple: There are no lighter particles that have an equivalent electric charge. This means they must be conserved. They could decay only by neutralizing an equivalent positive charge. In fact, if they interact, they both transform to gamma radiation. They can’t neutralise a proton as they lack the energy to make the pair into a neutron.
The second question is harder to answer. The short answer to the second question is that I don’t know what causes matter. While the Higgs mechanism may be proposed to answer this question I believe there are still things we don’t know about them.